Automated testing systems for integrated circuit devices are well known to those skilled in the art. Such systems may be used to test for a number of different types of faults. One common fault of interest for testing is the so called “stuck-at” fault in which the output of a logic circuit is persistently stuck in a certain logic state regardless of change in input. The detection of such stuck-at faults is of significant interest with respect to clock circuits, such as clock trees and clock generators, as the overall circuit operation will typically fail in the absence of a controlling clock signal. There is a need in the art for test circuitry operable to detect, for example by implication, a stuck-at fault condition of a buffer circuit within a clock tree.